For commercial polymers, the problems of flame and smoke are of tremendous importance. In particular, poly(vinyl chloride) (PVC) is a high-volume plastic that is inherently nonflammable and yet gives rise to both smoke and flame during fires. The generally accepted mechanism for the combustion of PVC involves dehydrochlorination that produces polyene segments that cyclize and then pyrolyze to generate volatile aromatic hydrocarbons. These hydrocarbons undergo vapor-phase combustion that gives rise to smoke and adds enthalpy to the combustion process (R. D. Pike et al., Macromolecules (1997), 30, 6957).
A new strategy, based on reductive coupling, has been introduced to interrupt the process of dehydrochlorination in PVC (W. H. Starnes, Jr. et al., Polym. Degrad. Stab. (2003), 82, 15). In dehydrochlorinating PVC, one of the polyene termini is generally recognized as being an allylic chloride moiety. Model-compound studies have shown that allylic chloride molecules can be reductively coupled to form new carbon-carbon bonds through the use of freshly reduced, high-surface-area metal films or powders. Because of its relative stability in reduced states, copper is a very active reductive coupling promoter.
In an effort to identify suitable copper-based precursor complexes that could be used as PVC additives, both copper(I) complexes and copper(II) complexes have been studied. The relative advantages of Cu(I) are its very facile reduction, its limited thermal stability, and particularly its lack of color. However, Cu(I) complexes often are too unstable to survive the requisite processing conditions. Thus, during PVC processing, many of these substances undergo partial decomposition, resulting in undesirable color and potentially impairing mechanical properties. On the other hand, Cu(II) salts are highly stable and generally would not be expected to suffer breakdown during processing or compounding. Unfortunately, all Cu(II) salts are colored, limiting their potential application in industrial processes.
Fire retardants and smoke suppressants are widely used in PVC formulations, and many different compounds are used for this purpose. In U.S. Pat. No. 4,053,455, Kroenke describes the use of amine molybdates as smoke suppressants in PVC polymers. One of the most commercially recognized materials for smoke suppression in PVC is ammonium octamolybdate (AOM). In U.S. Pat. No. 6,355,277, Day describes methods for improving smoke suppression in PVC formulations by incorporation into the polymer of (1) a complex of a cuprous halide and a phosphite, and (2) an amine molybdate which contains no residual molybdenum trioxide.
Nevertheless, fire and smoke hazards remain substantial, and there is a need for improved fire retardants and smoke suppressants.